1. Field of the Invention
The present invention relates to touch sensors and, more particularly, to force sensing touch location devices for accurately locating touch forces that include tangential force components.
2. Related Art
The ability to sense and measure and/or locate a force applied to a surface is useful in a variety of contexts. As a result, various systems have been developed in which force sensors are used to measure properties of a force (referred to herein as a “touch force”) applied to a surface (referred to herein as a “touch surface”). In response to the touch force, force sensors typically generate signals that may be used, for example, to locate the position on the touch surface at which the touch force was applied. A number of particular implementations of this approach have been proposed, such as that described by Peronneau et al. in U.S. Pat. No. 3,657,475.
Such touch location is of particular interest when the touch surface is that of a computer display, or that of a transparent overlay in front of a computer display. Furthermore, there is an increasing need for small, lightweight, and inexpensive touch location devices due to the proliferation of mobile and handheld devices, such as personal digital assistants (PDAs). Such touch location devices may be built with touch sensors based on a number of possible technologies, such as the force principle just mentioned, as well as capacitive, resistive, acoustic, and infrared technologies. Various features of prior art touch location devices and of force-based touch location in general are described in more detail in the commonly owned and concurrently filed patent application entitled “Method and Apparatus for Force-Based Touch Input.”
One problem with conventional touch location devices is that they typically misreport the location of touches that are not applied straight against the touch surface. In other words, if a user touches a touch surface with a finger or a stylus applying force at an angle other than 90 degrees to the touch surface, the location of the touch is typically misreported. The force applied by such an “angled touch” includes two force components: a component that is perpendicular to the touch surface (the “perpendicular component”) and a component that is parallel to the touch surface (the “tangential component”). Misreporting of the touch location is typically caused by the effects of the tangential force component that are sensed by one or more force sensors in the touch location device. Tangential force components of a touch force are also referred to herein simply as “tangential forces.”
For example, tangential forces may be transmitted to a sensor through a supporting sensor attachment, even though the sensor is not intended to measure such tangential forces. A pattern of forces and moments required to maintain equilibrium will be generated within the touch device by the tangential force, and aspects of this pattern may tend to pass in combination through any sensor that transmits portions of the tangential force itself. Errors in touch location may occur if the force sensor is sensitive to this combination.
Various solutions have been proposed to the errors introduced by tangential forces. For example, some systems place the force sensors somewhat behind the touch surface and simply tolerate the resulting error caused by tangential forces. Another approach has been to physically place all of the relevant touch-sensitive structure (e.g., force sensors and sensor assemblies) close to the touch surface, thereby reducing the impact of tangential forces.
Another approach has been to place explicit pivots in the touch plane for each force sensor support, as disclosed in DeCosta, U.S. Pat. No. 4,355,202, entitled “Mounting Arrangement for a Position Locating System.” Yet another approach has been to measure additional degrees of freedom of the touch force, such that the tangential component of the touch force may be corrected for, as disclosed in Roberts, U.S. Pat. No. 5,376,948, entitled “Method of and Apparatus for Touch-Input Computer and Related Display Employing Touch Force Location External to the Display.”
All of these prior art approaches have drawbacks. Ignoring tangential error may be unsatisfactory in precision applications, such as handwriting recognition, even when rearward sensor displacement is no more than the thickness of a 1 mm overlay. Removing tangential error by placing the sensors in the plane of touch may be unsatisfactory for reasons of device width and thickness. Pivoting the force transmission in the plane of touch, as disclosed by DeCosta, limits tangential error, but fails to address device width and thickness problems. Measuring all degrees of freedom, as disclosed in Roberts, limits tangential error, but requires additional space and additional complexity somewhere within the device.
Thus it is seen that the prior art fails to teach how force sensors may be located behind the plane of touch without either excessive error, excessive bulk, or excessive cost. Because angled touches (i.e., touch forces having non-zero tangential components) may occur frequently in a variety of applications, it is desirable that the location of touch on the touch surface be reported accurately by the touch location device, even if the touch is applied at an angle. Furthermore, it is desirable for such accurate location of angled touches to be achieved in conjunction with both planar and non-planar touch surfaces and using a variety of kinds of force sensors.